The present invention relates to a battery pack of which a battery element is coated with an outer jacket member.
In recent years, as portable electronic devices such as note-type personal computers, mobile phones, and personal digital assistants (PDAs) have been widespread, lithium ion batteries that feature high voltages, high energy densities, and light weight have been widely used for their power sources.
When a battery uses liquid electrolyte, a problem of which the liquid electrolyte tends to leak out of the battery takes place. To solve this problem, a lithium ion polymer secondary battery that uses a gel type polymer film of which polymer is soaked in nonaqueous electrolyte solution and a lithium ion polymer secondary battery that uses solid electrolyte have been practically used.
The lithium ion polymer secondary battery has a cell structure. The cell is composed of a battery element and an outer jacket member made of an aluminum laminate. The battery element is composed of a cathode, an anode, and polymer electrolyte. The battery element has electrode terminals that extend from the cathode and the anode. The battery element is coated with the outer jacket member. The cell and a wiring board on which a circuit portion is mounted are contained in a box-shaped plastic mold case composed of an upper case and a lower case. Japanese Patent Application Laid-Open Publication No. 2002-260608 describes an example of such a lithium ion polymer secondary battery.
In the structure of related art of which a battery element is contained in a mold case, the wall thickness of the mold case is around 0.3 mm to 0.4 mm. When a fixing double-adhesive tape and tolerance are considered, the thickness of the cell increases by around 0.8 mm to 1 mm. In addition, the shape that allows both the upper mold case and the lower mold case of the cell to be ultrasonic-adhered is required. To do that, the wall thickness increases by around 0.7 mm. As a result, the volume of the battery pack increases 1.3 to 1.4 times compared with the volume of the cell.
In addition, when the battery cell is contained in the mold case, the heat radiation will deteriorate. As a result, the battery performance will lower and/or generate heat.